Why did the memorial altar east of the Jordan Nearly cause a civil war?

ANSWER

The Israelites west of the Jordan thought it was an unauthorized altar to God

Joshua 22 :

5 When they went to Gilead—to Reuben, Gad and the half-tribe of Manasseh—they said to them: 16 “The whole assembly of the LORD says: ‘How could you break faith with the God of Israel like this? How could you turn away from the LORD and build yourselves an altar in rebellion against him now? 17 Was not the sin of Peor enough for us? Up to this very day we have not cleansed ourselves from that sin, even though a plague fell on the community of the LORD! 18 And are you now turning away from the LORD? “‘If you rebel against the LORD today, tomorrow he will be angry with the whole community of Israel. 19 If the land you possess is defiled, come over to the LORD’s land, where the LORD’s tabernacle stands, and share the land with us. But do not rebel against the LORD or against us by building an altar for yourselves, other than the altar of the LORD our God. 20 When Achan son of Zerah was unfaithful in regard to the devoted things, did not wrath come on the whole community of Israel? He was not the only one who died for his sin.’”
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